Which type of muscle tissue is involuntary and forms the walls of the heart?

Cardiac muscle is the correct answer as it is specifically designed to function involuntarily and is found exclusively in the heart. This unique type of muscle tissue is characterized by its striated appearance, similar to skeletal muscle, but operates under different control mechanisms. The involuntary nature of cardiac muscle means that it is not consciously controlled; instead, it is regulated by the autonomic nervous system and intrinsic pacemaker cells that coordinate heartbeats.

The cardiac muscle's structure also allows for efficient contraction and communication between cells, permitting the heart to pump blood effectively throughout the body. This tissue has intercalated discs, which facilitate the rapid transmission of electrical impulses, ensuring synchronized contractions necessary for maintaining a steady heartbeat.

While skeletal muscle is voluntary and is responsible for body movements, and other types such as visceral muscle refer to smooth muscle found in organs like the intestines, these are not relevant to the heart's function. Thus, the specific properties and location of cardiac muscle make it the appropriate answer in this context.